Process for dehydrating aliphatic acids by a combined extraction-distillation method



Jan. 6, 1942, D. F. oTHMER 2,269,163 PROCESS FOR DEHYDRATING ALIPHATIC ACIDS BY A COMBINED EXTRACTION-DISTILLATION METHOD v Filed April 7, 1938 INVENTOR BY l `7d' p8 ATTORNZS Patented en. 6, 1942 TENT OFFICE PROCESS FOR DEHYDRATING ALIPHATIC ACIDS BY A COMBINED EXTRACTION- DISTILLATION METHOD Donald F. Othmer, Brooklyn, N. Y.. assgnr, by

mesne assignments, to Tennessee Eastman Corporation, Kingsport, Tenn., a

Virginia.

corporation of Application April 7, 1938, Serial No. 200,746

Claims.

This invention relates to a process and solvents for the dehydration of aqueous solutions of lower aliphatic acids, and more particularly to a process for the production oi substantially concentrated or glacial acetic acid from aqueous acid solutions, wherein heating requirements are minipayed 1n large quantities mdustriauy. These acids are in many instances obtained as by-products* or in their manufacture are produced in the form of dilute aqueous solutions. Before the acids are suitable for certain uses they must be dehydrated or concentrated. vAs pointed out in my copending application, various distillation processes for dehydrating aliphatic acids have been suggested. Since many of the processes involve rather extensive distillation treatments, one

o f the largest factors of cost is the heat input.

required. v

Certain investigators have developed processes designed to reduce the heat consumption. For example, it has been proposed to distill aqueous solutions azeotropically and pass the azeotrope vapors in heat exchange with an independently prepared extract, thereby producing Atwo independently concentrated acid residues.

It has also been proposed to partially azeotropically dehydrate the acid and then extract the partially dehydrated acid, subjecting the resultant extract to heating by means of the aforementioned azetrope. Other processes have been suggested in the prior art employingbenzene and the like as entrainers.

It has been suggested 'that various ketones alone might be used as withdrawing agents. Also, various mixtures-of ketone with benzene, esters or ethers for use as entraining agents has been suggested. I have discovered a novel agent and method of operation distinguishing from the prior art which are more eilicient in certain respects than prior art entrainers and processes and possess other advantages.

This invention has for one object to provide a process for the dehydration of aqueous solutions containing at least one aliphatic acid. Another object is to provide a combination process for the dehydration of aqueous solutions containing at least one aliphatic acid, which process is more economical of heat and otherwise more advantageous than certain prior art processes. A still further object is to provide a combination process for concentrating laqueous aliphatic acid solutions wherein relatively pure concentrated acid may be recovered. A still further object is to provide a process for the concentration of lower aliphatic acids which may be appliedto acids from various sources such as aqueous acid solutions from `synthetic acid production, aqueous acid solutions from fermentation processes, pyroligneous liquor and various other sources of aqueous acid. A still further object is to provide a combination process for dehydrating aqueous`aliphatic acid solutions which process includes straight distillation in combination with an azeotropic distillation employing vaporous heat from the straight distillation. Still another object is to provide a process for concentrating lower aliphatic acids which is continuous.

A further object is to provide novel entraining agents which may be employedin processes for the dehydration of lower aliphatic acids. Another object is to provide a novel mixed ketone entraining agent. Still another object is to provide a combination entraining agent which may 'be used both as an extracting agent and as an entraining agent. Still another object is to provide an agent which is particularly adaptable for use in the treatment of aqueous solutions containing lower aliphatic `acids in high concentration. Still another object is to provide a process for the dehydration of aqueous solutions containing lower aliphatic acids which process includes a particular type azeotropic distillation. Other objects tvill appear hereinafter,

I have found an improved process wherein very high heat economy maybe obtained. I have found that it is advantageous that the water be removed from a dilute aliphatic acid solution by cold extraction and from a more concentrated solution by azeotropic distillation, provided the proper agents and process steps are employed.

For a more complete understanding of my invention, reference is made to the attached drawing, which diagrammatically represents an ap'- paratus set-up which might be 'employed for carrying out my invention.

In the attached drawing, l represents a feed tank or other container for holding the aqueous aliphatic acid solution to be treated. This tank is connected to an ,extraction device 3, by means of conduit 2. f

It is, of course, desirable that an ecient unit or units be employed. For example, this device might comprise an open column (with two liquid phases and droplets of the heavier liquid falling throughl the upper orlighter liquid Phase, and

droplets of the lighter liquid rising through the lower or heavier liquid phase). Buch a deviceis relatively simple and economical to construct and operate. Two or three 60-80 ft. extractora would beemployed.

Thefeedtank I isalsoconnectedbymeansof another conduit 4 to a pre-evaporator or evapohoused by an external chamber adapted for con-4 taining liquid. A preferred type of device would be the outside vertical calandria type having either natural or forced circulation. Because of the high rate of heat transfer secured by this type of evaporator substantially full advantage of the low temperature drop between the vapors giving up the heat and the liquidvbeing heated, could be obtained. It is also possible to employ several evaporating chambers in series or other arrangements.

The evaporator II is connected by conduit I2 to a decanter I3 which in turn is connected through conduit I4 to column 8. The decanter is also connected by conduit I8 to an auxiliary column I1 which is in turn connected through conduit I8 to the condenser evaporator II.

lThe condenser evaporator feeds through the plurality' of conduits 2I, 22 to intermediate sections of another column 23. This column is similar to column 8 hence, requires no further description excepting to point out that both columns 8 and 23 have heating means as at 24 and 23.

I The condenser evaporator II is connected by conduit 21 to the extraction device 3 already described.

Column 23 is also provided with several con-ruig,

annales hereinafter, it is possible to evaporate the solvent layer substantially completely. It is to be noted, however, in my process, that the unevaporated liquid goes through conduit 22 to the extract column 23.l

The vapors from the device II pass to an intermediate section of column 23, where they are rectiiied and a distillate out through the vapor outlet conduit 23. This distillate is condensed'at 34 and to the decanter 33. Because of the type of solvent employed, only a small amount of water is brought over and separated from solvent in the decanter. The water layer in the decanter may be passed through conduit 32 into the water column along'with that .from the extractor for stripping' solvent therefrom. The solvent layer from the decanter 33 is divided, a minor part being returned through 3I for reflux, the balance being returned through conduit 33 to the cold extraction.

Referring again' to Vextract column 23, as already indicated, another intermediate section thereof receives the feed through conduit 22 of excess unevaporated liquid from the condenserevaporator II. A Howeven by virtue of the base heater 24, complete separation of all the low boiling point solvent is assured. Consequently, it will be noted that not only does column 23' discharge an -aqueousacid but this aqueous acid is discharged .through conduit 4I into an intermedi- -ate section 42 of the azeotropic column 3. It will be observed in my process that it is not necessary to remove water in the extract column 23, excepting that which is removed in separating the solvent from the vapors fed to the unit. This is of some importance, because solvents employed for extraction sometimes have poor entraining capacity for azeotropic distillation. Hence, in my process most water which is removed by distillation, is removed by means of a particular type highly efiicient azeotropic agent by procedure hereinafter described.

That is, in my process the strengthened but still aqueous acid from the bottom of column 23 duits 28, 29, 3|, 32,33 and the condenser 34and decanter 38, which serve to permit carryingout of distillation operations as well as to inter-connect the devicevwith extractor 3. The aqueous AVoutlet 31 leads to a solvent stripping column 33 of conventional construction.

One feature of particular importance which the apparatus set-up embraces is that extract column 23 is directly connected by means of conduit 4I to an intermediate section 42 of the azeotrope column 8.

As indicated, the apparatus has been shown more or less diagrammatically and in ilow sheet arrangement, and certain modications may be introduced therein. For example, if desired, partial heating of the low boiling solvent strippingl ,evaporated in the condenser evaporator I I above the azeotropic column. By employing suitable solvents, as will be described in further detail -be given up in the evaporator condenser II.

`is run as a liquid fed throughV conduit 4I to the -azeotropic column 3 using a highv boiling point emcient entraining agent in the particular manner to be described. This acid is then substantially or completely dehydrated in column 3.

The azeotropic vaporous mixture from column 3' is condensed in the condenser-evaporator and separated in the decanter I3. If desired, this decantation may be carried out at a temperature aV few degrees below the boiling point of the azeotrope. The water layer from the decanter flows through conduit I3 to column I1 where any solvent is stripped out and returned through conduit I8 to join the main azeotropic vapors in condenser-evaporator II.

It is apparent from the preceding that the balance between the extraction procedure and the azeotropic procedure depends upon the amount of heat passed in the condenser evaporator II, vwhich in turn depends on the amount of water removed in the azeotropic distillation 8 which in turn depends to some extent on the strength of liquid from the b ase of extract column 23, -returned through conduit 4I. Therefore, if a comparatively dilute liquid comes from the extract column, more heat will be required by the azeotropic column 8 and more heat will In the preferred operation of my process the amount of heat available in the azeotropic mixture or vapors passing out through conduit 9, should be.

not substantially greater than that required to evaporate-the extract in the condenser evaporator H. I will point out in detail hereinafter wherein I am able to control this factor to some extent. If-the heat is greater an additional condenser may -be inserted in lines 9 or v12, for example.

On the other hand, if a more concentrated liquid comes from the base of extract column 23, it is possible that the heat required to evaporate the extract in the condenser evaporator will be less than that supplied to the condenser evaporator by the vapors passing out through conduit 9. I have found that in this case I am able to controlthe relative heat ratios by feeding an additional amount of the original aqueous acid solution through conduit 4 to the pre-evaporator 6 from which the acid reaches column 8 through the conduit 1. The removal of the water in this additional-aqueous feed requires an additional amount of heat in the azeotropic distillation which increases the amount of heat passing to the evaporator condenser to an amount suillcient to evaporate the extract, or that part of the extract which it is desired to evaporate. That is,

I have found by controlling the flow of aqueousV acid through 4 either directly to the column or indirectly through the pre-evaporator 6 that I am able to more or less control and balance the heat requirements in my process.

While as I have pointed out, the most effective balance between the extraction part of the proc ess and the azeotropic distillation is obtained when the water to be removed in the azeotropic distillation requires that amount of heat in the azeotropic mixture which, when transferred in the condenser evaporator Il, will just evaporate the amount of extract desired evaporated, such conditions may be altered in commercial use. That is, this balance and interrelationship may be affected yby Various factors such as (l) strength of dilute acid to be handled, (2) partition coeiiiient of acetic between low boiling solvent and water, (3) water solubility in low boiling solvent at different amounts of acetic, (4) efficiency of extractor and extraction operation, (5) boiling point of low boiling solvent and its azeotropic mixture (if any) with water, (6) efficiency of condenser-evaporator in its evaporative function, (7) azeotropic mixture ratio of the high boiling solvent, (8) mutual miscibility traction apparatus 3 and my novel mixed ketone entrainers to be described hereinafter employed in the azeotropic distillation column B.

I have lfound that mixed ketone agents ccmprise entrainers which are particularly useful in portions are set forth for the purposes of illustration and may be varied.

I have carriedl or had carried outv underl my direction a number of tests employing the abovementioned mixture of di-isopropyl and ethyl isopropyl ketone and have found this ketone mixture to have an excellent withdrawing power bringing over almost one part of watenfor four parts of ketone distilled. The azeotropic boiling point is approximately 89 C. at a normal barometer. These figures would vary somewhat in a commercial installation because there would be a slight tendency to lose the lower ketone by evaporation loss and thus to make the azeotropic boiling point higher and to bring over a greater amount of water in the azeotropic mixture.

This material is an excellent withdrawing liquid ,also because it has a very good extraction coefficient for acetic acid in contact with water, which is unique in that it improves with higher acid concentrations. It tolerates a large amount of acid, which means that it could be used as an extracting agent for relatively high acid concentrations. A table follows showing the relation of acid strength in each layer.

of high boiling solvent with water, (9) partition i coecient of high boiling solvent with water for acetic acid and other factors which determine its azeotropc efficiency.

However, as pointed out, I am able to control my process to some extent by control of the quantity of aliphatic acid fed to the azeotropic column 8 by way of the pre-evaporator.

The feed from the extract column 23 through conduit 4| may, of course, be passed into the pre-evaporator 6 rather than to the intermediate section of the column as at 42. However, this is not necessary because the preceding treatment in the extract column, condenser-evaporator and extraction, has eliminated the bulk of the salts or other impurities which it might otherwise be necessary to remove in pre-evaporator 6.

Acetic acid, grams per 100 cc.

` Water layer Ketone layer ter, less water is returned-to the azeotropic column and less ketone is discharged to the stripping column if an apparatus as shown in Othmer 2,028,800 is employed.

As has been pointed out, the acid solvent used The entraining agent employed in column 8, whether it be my preferred mixed ketones or some other agent such as ethyl hexyl ether or other high boiling point ether is preferably employed in the following special manner. That is,

the mixture of acid and water fed to column I through conduits l and 4| is azeotropically, distllled with the agent so that the bulk. but not all of the water. is removed in the azeotropic section of the column. served that in my process preferably all of the -water (even that separated in the lower part of the column by straight distillation) works its way up through the azeotropic ,section and isthus finally discharged by azeotropic distillation.

' .ketone agentA through Il in flowing down column l, will remove the acid from the vapors passing upwardly in the column.

The water remaining in the acid after the bulk of the water has been removed by azeotroplc distiliation, is more or less continuously eliminated from the acid by straight distillation in the bottom of column 8. The vaporous heat from this straight distillation is transmitted to the azeotropic distillation and consumed therein.

Or, if it is desired, my novel ketone mixture may be employed in other type extraction or dis- That is, I have found that my novel ketone mixtures have considerable value for use in azeotropic distillation alone for the production of Adehydrated aliphatic acid. For example, a mixture of the ketones may be supplied to the decanter or other Yinlet to a continuous azeotropic distilling system. A portion of the ketone would be per- It will be further ob,

tillation processes for dehydrating aliphatic acids. ,I

yters Patent of the United States is:

mitted to flow back through the upper part of the distillation column. Aliphatic acid (preferably vaporous) to be concentrated would be supplied to an intermediate section of the column at some point below the aforementioned ketone return. This is advantageous because as I have already pointed out, my ketone mixtures tend to extract the acid from the vapors arising in the column.

A vaporous mixture comprising the several ketones and water would be removed from the head of the column, condensed and passed to the decanter. In the decanter the several ketones form a distinct layer, thereby separating from water and this ketone layer may be returned to the column as already described. The water layer may be withdrawn to an auxiliary column where any ketones entrapped or otherwise lost therein may be removed prior to discharging the water to waste. These removed ketones are preferably conducted through the condenser and backv to the decanter from which they are returned to the column. By this procedure the column is prevented froni becoming unbalanced and'operation is otherwise improved.

Preferably the amount of ketones employed is such that the lower portion of the column comprises only aliphatic acid and water. This may be accomplished by employing insufiicient ketone mixture in the system to furnish a content in the lower portion of the column. The water remaining in the acid may be distilled therefrom in a manner such as described with respect to column 8.

The dehydrated acid would preferably be withdrawn from the base of the column in a vaporous condition. If the procedure described is employed, a relatively pure acid uncontaminated by,

v the ketone mixture will be recovered.

From the preceding it is apparent that I have provided a novel process wherein heating requirements may be reduced to a minimum. This is. possible because in my process substantially all of *the water removed by distillation is removed using a highly eiiiclent entraining agent. The heat of this mixture is substantally recovered. If desired, the decantation in 13 may take place at a temperature of a few degrees below the condensation point of the azeotrope. Also. in my process, the heat of the `hot aqueous acid base of column 23 is not lost because said acid is fed to distillation column 8. Inaddition. there are various other features disclosed herein, all contributing to rendering my process particularly emcient. While I have described certain procedure for my preferred embodiment, inasmuch as I have found such procedure to produce most optimum results'. itis to be understood that my invention embraces wider aspects. For example, as described, my novel ketone mixtures may be employed in various ways in the dehydration of aliphatic acids. Although I have shown an extraction column and various other apparatus units, it is to be understood that other devices may be employed. Hence, I do not wish to be restricted in my invention excepting insofar as may be necessitated by the prior art and by the spirit of the appended claims.

What I claim and desire to be secured by Let- 1. A 'process for'V dehydrating aqueous acetic acid solutions, which comprises forwarding such solutions in the vaporous state into an intermediate section of a distillationV column containing a mixture of a symmetrical ketone with an unsymmetrical ketone, vaporizing symmetrical ketone, unsymmetrical ketone and water from the column, separating the symmetrical and unsymmetrica] ketonemixture from the water and returning this ketone mixture to the distillation column at a point substantially above the vaporous feed, whereby said lfetone mixture ilows countercurrent to the vapors v'in the column undergoing distillation, and withdrawing the dehydrated acid from below the vaporous feed.

2. A process for dehydrating aqueous solutions containing at least one lower aliphatic acid therein, which comprises conducting such solutions in the vaporous state into an intermediate portion of a distillation column, said column containing a mixture of 50% to 80% di-isopropyl ketone with 15% to 40% ethyl isopropyl ketone as a water-withdrawing agent, vaporizing di-isopropyl ketones, ethyl isopropyl ketone and water from the column, separating at least a part of the di-isopropyl ketone and ethyl isopropyl ketone from the water and returning the` separated ketones to the distillation at a point substantially above the vaporous feed, whereby the returned ketones flow countercurrent to the materials in the column undergoing distillation, and withdrawing dehydrated acidfrom a point below said vaporous feed.

3. A process for dehydrating aqueous solutions containing at least one lower aliphatic acid, which comprises passing such solutions in a vaporous condition into a distillation column containing a mixture of a symmetrical ketone with an unsymmetrical ketone, vaporizing symmetri- V cal ketone, unsymmetrical ketone and water the vaporous feed, whereby said ketone, mixture flows countercurrent to the vapors in the column undergoing distillation, and withdrawing thedehydrated acid from below the vaporous feed.

4. A process for dehydrating aqueous solutions containing at least one lower aliphatic ,acid

therein, whichvcomprises conducting such solutions in the vaporous state into an intermediate-l portion of a distillation column', said column containing a mixture principally of methyl propyl ketone with di-ethyl ketone as a water-withdrawing agent, vaporizing methylpropyl ketone.

di-ethyl ketone and water from the column, sep- 5. A process for dehydrating aqueous aliphatic acid solutions which comprises conducting such solutions in a vaporous state to an intermediate section of the distillation system, the system containing a mixture of a symmetrical ketone with an unsymmetrical ketone, distilling oil symmetrical ketone, unsymmetrical ketone and wa.- ter from the distillation system, passing at least a.v part'of the vapors so ldistilled of! in heat exchange with a cold mixture of acid, solvent, and

water for cooling said vapors, separating symmetrical and unsymmetrical ketone vapors from water and returning this separatedketone mixture to the distillation at a point substantially above said vaporous feed, whereby said ketone mixture flows .countercurrent to the components in the column undergoing distillation and withdrawing dehydrated acid from below said vaporous state. Y

, DONALD F. 

